A A LOW-COST HYDRAULIC CYLINDER '■■f0Hyn&. ■>,-t: : .'V:r '■l r .f-'M- i ;-.;- : Jr«J$f-Ot ','<■■&/&<&£? WEST VIRGINIA UNIVERSITY AGRICULTURAL EXPERIMENT STATION BULLETIN 493T, MAY 1964 Digitized by the Internet Archive in 2010 with funding from Lyrasis Members and Sloan Foundation http://www.archive.org/details/lowcosthydraulic493phil LOW-COST HYDRAULIC CYLINDER Ross A. Phillips WEST VIRGINIA UNIVERSITY AGRICULTURAL EXPERIMENT STATION THE AUTHOR Ross A. Phillips is Assistant Professor of Agricultural Engineering and Assistant Agricultural Engineer in the Agricultural Experiment Station. This study was part of a North- east Regional Project— "The Mech- anization of Forage Crop Harvest- ing, Processing, Storing and Feed- ing," NE-13, a cooperative study involving agricultural experiment stations in the Northeast Region and supported in part by regional funds. West Virginia University Agricultural Experiment Station College of Agriculture, Forestry, and Home Economics A. H. VanLandingham, Director Morgantown A LOW-COST HYDRAULIC CYLINDER H YDRAULIC cylinders are used extensively wherever linear motion with high force and low velocity is required. This re- quirement was encountered in designing an experimental hay drier where a linear motion through a distance of about five feet was needed. Desired forces were estimated to be around 5,000 pounds, to be applied on the end of a bale of hay. Various mechanical devices were considered but all were either too cumbersome or inter- fered with the working space near the bale of hay. Room was available for a long cylinder, and cost estimates were obtain- ed for a suitable commercial cyl- inder. These were considered excessive for the job. The cyl- inders were also excessively heavy for convenient handling. Assuming that other researchers must have similar needs, the fol- lowing objectives were estab- lished for building a cylinder in the laboratory: (1) light weight, (2) low cost, (3) the ca- Ross A. Phillips pacity to produce the desired forces through the required dis- tance, and (4) a reasonable life expectancy but which could be a much shorter use period than normally specified for commer- cial cylinders. Cylinders for Agricultural Applications Most modern tractors have built-in hydraulic systems. These operate a special cylinder or cylinders integral with the trac- tor. Many systems have con- nections to operate a remote cyl- inder. The cylinders integral with the tractors have adequate production volume and use to justify special designs and honed finishes. The remote cyl- inders frequently have limited use on any one machine, due to seasonal operations, and are built in two standard sizes so that one cylinder may be used in numerous places. This justi- fies a more expensive cylinder and reduces costs due to the greater volume of production. The two sizes have 8 inch and 16 inch strokes with extended lengths of 28.5 inches and 47.5 inches and such diameters that the rate of operation is within specified limits. Thus, a remote cylinder for a tractor with a hy- draulic pump having a given volumetric displacement would not be interchangeable with a tractor having a hydraulic pump with a considerably different volumetric displacement. Most machines that need hydraulic controls can be designed to comply with these standard cyl- inders. Low cost special cyl- inders would find additional uses where the standard cyl- inders will not fit into a design. Also, these cylinders could open a new area for farm me- chanization in which a tractor is not involved. Equipment in- tegral with a structure is one area where this would apply. Industrial Applications Hydraulic cylinders of almost any size and shape can be found in various industrial uses. These have honed working surfaces. Frequently the working surfaces have noncorrosive and hardened platings which contribute to the long life of the cylinder. Special molded packings are used in conjunction with these excellent finishes with the result of trouble-free performance for millions of cycles. These fine finishes are very expensive, but quite desirable where continued use makes them economically feasible. Such extended lasting qualities are not necessary with most agricultural equipment. A specific installation may have less than 15,000 cycles in a 10- year period and be used as much as four times daily. Thus, a well made cylinder in a similar installation would become obso- lete long before it would wear out. Efficiency of Speed Force Ratios High-pressure pumps operate at favorable efficiencies which may be as high as 95 per cent. Cylinders also have good ef- ficiencies. Linear motion at any relatively low velocity may be obtained in one step from the pump to the cylinder. This re- quires the proper diameter of cylinder for a given pump ca- pacity. By similar reasoning, any desired force may be ob- tained with a given amount of power. Major power losses occur in the fluid flow between the pump and cylinder. These may be con- trolled and held within desired limits by use of adequate lines. The relative efficiency of a mechanical and a hydraulic sys- tem to provide a high mechani- cal advantage is much in favor of the hydraulic system. Specialized and Experimental Use The design of this cylinder was such as to provide for gen- eral application in a variety of uses. Facilities available in u Hi O different shops may require al- teration of the design to some extent. Machine work was held to a minimum. Materials used are readily available in most locations and in small quanti- ties. The objectives of (1) light weight, (2) low cost, (3) desired capacity, and (4) reasonable short life were all incorporated into the design. The final re- sult is illustrated in Figure 1. Dimensions can be varied, with- in structural limits, to any de- sired size. Lightweight was accomplish- ed by using aluminum. Extruded tubing and extruded solid stock was supplied by the Aluminum Company of America. The tubing alloy was 6062-T6. The solid stock alloy was 2017-T4. Mate- rial cost for this would be high per pound, but reduced machine work would cancel the material cost. The total estimated cost of materials was $40. Each alu- minum alloy had a yield point of 40,000 psi. A working stress of 10,000 psi was used for design; however, for the specific appli- cation of the cylinder being built, this never became critical. Cylinder Body Extruded finishes of the tube were used to reduce machine work. The inside tolerances were within + .001 as determined with a micrometer. A wall thick- ness of 0.250 inch was used to provide adequate rigidity. This cylinder wall would withstand a hydraulic working pressure of Extruded Finish — v 2+-I6N- FIGURE 2. Cylinder Body. 2,500 psi, and the threaded ends were designed for this pressure. Threading and facing the ends was the extent of machine work on the body (Figure 2). Cylinder End The cylinder end away from the plunger was machined from solid aluminum stock (Figure 3). The "O" ring seal was placed so that no lateral pressure was on the machined part of the cyl- inder body. Some shops would prefer welded construction at this end, but threaded assembly had the advantage of providing for the removal of both ends for inspection of the cylinder body. Close tolerances were required only for the "0" ring groove. I Drill— \ +NPT Packing Gland Solid stock was used for this piece. Tubing with 1.5 inches inside diameter would have been as satisfactory as the solid stock. External threads and the "0" ring groove of this piece were identical with those of the cylinder end. The oil passage along the plunger rod was de- signed to have a cross section of the same area as available in one- quarter inch pipe fittings. Space was provided for one- quarter by one- quarter inch linear packing around the cyl- inder rod and clearance for the packing nut to compress this packing in the packing gland (Figure 4). 2i-l6N l Drill - FIGURE 3. Cylinder End. 4 FIGURE 4. Packing Gland. Packing Nut Solid stock was again used, although tubing with 1.5 inches inside diameter would have been as satisfactory. Tolerance be- tween the plunger rod and the packing nut was held close so that the nut would act as a wiper to keep foreign matter away from the packing. The close fit also reduced the tendency for the packing to extrude (Figure 5). 2g-l6N- FIGURE 5. Packing Nut. -i-IONC Plunger Rod Machine work was held to a minimum by using cold rolled steel and machining only the ends of the rod (Figure 6). A diameter of 1.5 inches provided a favorable 1/r ratio of less than 170, assuming the cylinder body to be completely supported. A 1.25-inch diameter would give a ratio of more than 200 when the plunger was extended. American Institute of Steel Construction working stresses allowed a load of 9,150 pounds with the plunger rod fully extended and with no eccentric loading. Since the maximum design load of the cyl- inder was 7,850 pounds, this would allow for a small eccen- tric load. Such loading could possibly be caused by friction Cold rolled finish FIGURE 6. Plunger Rod. 5 in the pins of a flexible linkage, or by tolerances of fabrication. This design load was approxi- mately one-fourth of the ultimate load as calculated by Euler's column formula. When the cylinder body is not supported and the rodend is con- sidered, the maximum column length increases from 63.5 inches, used for the above calculations, to 142.75 inches. The ultimate load calculated by Euler's col- umn formula, P =flx 2 El where \% P = load ft = factor for end conditions = 1 for round ends E = modulus of elasticity , I = least moment of inertia in inches 4 1 = length of column (inches) would be 3,490 pounds, and inde- pendent of the fiber stress of the steel rod. The modulus of elasticity used for steel was 29 million pounds per square inch. An aluminum alloy (Alcoa 2017- T4) with a modulus of elasticity of 10.5 million pounds per square inch was also used forthe plung- er rod and had an ultimate strength of 1,265 pounds with the cylinder body unsupported. Obviously, these loads present great limitations for the cylinder under these particular conditions. However, for a long column the load increases as the inverse square of the length. As the cylinder contracts, the center of the column moves from near the end of the cylinder body where the plunger rod diameter con- trols the slenderness ratio, to- wards the center of the cylinder body. Thus, the ultimate load of the plunger rod would be approx- imately equal to the design load of the cylinder at an extension of 46 inches. Certain applica- tions could possibly be practical with the cylinder body unsup- ported if the actual load were always a reasonable amount be- low the ultimate load for any amount of extension. Plunger Assembly The plunger (Figure 7) was made relatively long to conform with standard practice. This helps to give stability when the FIGURE 7. Plunger. cylinder is extended. Tolerances for the outside diameter were held close to reduce tendencies for extrusion of the cups. The "0" ring was to seal the high pressure side from the low pres- sure side, and the location or type of this seal was not critical. The cup hub (Figure 8) on the FIGURE 8. Cup Hub. 6 FIGURE 9. Plunger Rod End. extension side of the plunger acted as a stop at full extension of the cylinder and gave protec- tion to the cup. The plunger rod supplied adequate protection for the other cup if the cylinder was contracted without the rod end FIGURE 10. Cup Hub. (Figure 9) which normally acted as a stop. The hubs (Figures 8 and 10) were sized to hold the cups firmly without excessive force and to allow for slight lateral flexibility. Leather water pump cups were used as they were readily available and the assumption was made that leather would produce as much erosion on the cylinder wall for test purposes as any cup mate- rial. After some use, a cup ex- pander was added to the cup on the nut side of the plunger. The assembly was held together by a standard th re e-qu arter inch course thread hex nut. Cylinder Performance The cylinder was mounted on the experimental hay drier for which it was designed (Figure 11). An automatic reversing valve, which is shown on the cylinder, kept the plunger cycling as long as fluid was supplied. A counter was connected to the reversing mechanism to obtain the number of cycles the cylin- der made. The pressure relief valve was never set above 1,000 psi since this gave the maximum force desired on the hay drier. The cylinder was supported by the connecting pins as a short cylinder would normally be supported. When the plunger was extended, the 1,000 psi gave a force (3,140 pounds) near the ultimate and the cylinder would deflect considerably when eccentricity was applied by a lateral hand push on the cylin- der. No failure was encountered. Supports in a later model of the drier consisted of the pin at one end of the cylinder and a clamp to hold the rod end of the cylin- der in place. The pushing plate was equipped with loose fitting guides. This gave considerably more safety from any side thrust that would accidentally be ap- plied to the cylinder. An aluminum rod was tried with this mounting, but exces- sive deflection occurred before the desired loads were reached. This deflection was at loads be- low those calculated for long column action, which indicated that eccentricity was introduced by the pin connections. Controls were manually operated and guides afforded enough support that loads were removed before exceeding the elastic limit of the plunger rod. Expansion of the cylinder due to pressure was calculated to be one-thousandth of an inch at 5,000 psi. No measurement of change was obtained up to the 1,000 psi at which the cylin- der was operated. Thermal ex- pansion for aluminum is twice that of steel or an increase of one-thousandth of an i n c h on the cylinder diameter for approx- imately every 40° F temperature rise. The average temperature rise encountered with one horse- power input was slightly less than 30 ° F and the micrometer measurements indicated one- thousandth inch expansion. Most packing materials would accom- modate this thermal change of dimensions as the aluminum plunger would expand at approx- imately the same rate as the cyl- , inder. Total operation of the cylin- der has been an estimated 20,000 cycles with varying pressures up to 1,000 psi. The counter was used only for some 300 cycles because the cylinder was re- quired to operate through a def- inite stroke to actuate the coun- ter. This hampered the practical use of the cylinder control valve. After one year's operation and approximately 2,000 cycles, the plunger cup used for extension was deformed so the lip of the cup did not seal on the cylinder wall. This was corrected with a cup expander. Measurements FIGURE 11. Hydraulic System for Pushing Hay Bales into an Experimental Drier. were made periodically with a micrometer on the inside diam- eter of the cylinder and on the diameter of the cold rolled steel plunger rod. Neither di ameter changed enough in the estimated 20,000 cycles to indicate a change of dimensions. The wear was less than five ten-thou- sandths of an inch. After a few cycles (less than 100) of opera- tion, wear marks were clearly visible on both the plunger rod and on the inside of the cylinder indicating a seating of the plunger cups and the plunger rod packing. The packing nut required fre- quent adjustment to prevent oil leakage around the plunger rod. The first adjustment after reas- sembly was required between 50 and 100 cycles and subsequent adjustments at approximately 500-cycle intervals. A graph- ite-filled asbestos packing was used. Other types might require much less attention. The main- tenance presented no difficulty in the research operations. Re- placement of either packing or cups did not appear necessary before at least 100,000 cycles of operation. One precaution was used in storing the cylinder. The cyl- inder was contracted to prevent corrosion of the cylinder rod. The adhering oil would not give adequate protection and rusting occurred after an exposed period of one month in an unheated building. The length of permis- sible exposure would vary con- siderably with weather condi- tions. Summary The aluminum hydraulic cyl- inder with a cold rolled steel plunger rod was satisfactory for the application for which it was designed. The extruded finish of the tubing and the cold rolled surface of the steel gave ade- quate performance with the plunger cups and packing used. Factors which should be ob- served in the design of similar cylinders are (1) the low mod- ulus of elasticity of aluminum where column action becomes critical, and (2) the high thermal coefficient of expansion where excessive temperature changes are involved and a close fit is required with steel. Seals must accommodate the dimensional changes of either pressure or temperature.